A new clinical entity termed Embolic Stroke of Undetermined Source (ESUS) was recently introduced by the Cryptogenic Stroke/ESUS International Working Group, which describes stroke patients for whom the source of embolism remains undetected despite thorough investigation; potential embolic sources include diseases of the mitral and aortic valves, the left cardiac chambers, the proximal cerebral arteries of the aortic arch and the venous system via paradoxical embolism. ESUS has been proposed as a potential therapeutic entity with a possible indication for anticoagulation, a hypothesis which is currently tested in two randomized controlled trials.